The Doomsday Algorithm It's the last day of Jan. or of Feb. that will do (Except that in Leap Years it's Jan. thirty-two). Then in the even months use the month's day And for odd ones add four, or else take it away, According to length, or simply remember: You only subtract for Septem. or November. Now to work out your Doomsdays the orthodox way, Three things you must add to the century day: Dozens, remainder, and fours in the latter (If you alter by sevens, of course it won't matter). In Julian times, lackaday, lackaday, Zero was Sunday; each hundred went back a day. But Gregorian four hundred are always a Tues. And centuries extra each take us back twos.